The chromizing of steels by gaseous method is well known. The formation of chromium-based diffusion alloys on the surface of steels has already been described in many patents. The transfer of chromium to the surface of the material to be treated is done by means of halides, which are the only compounds with chromium to exist in the vapor state at diffusion temperatures. The passage of the chromium into solid solution in the metal is effected by exchange between the chromium halide and the iron according to a reaction which, in the case of chlorides, can be written: EQU CrCl.sub.2 (g)+Fe(s).fwdarw.FeCl.sub.2 (g)+Cr(s) (1)
To obtain a sufficient diffusion, the reaction must be effected at high temperature and in the austenitic range, that is to say over 850.degree. C. for conventional steels. In steels whose carbon content is greater than 0.2%, the surface reaction of the carbon and chromium involves on the one hand the formation of a skin of chromium carbides and on the other hand a diffusion of carbon towards the surface. The surface skin is formed of two types of carbides, M.sub.23 C.sub.6, richer in chromium towards the surface and M.sub.7 C.sub.3, poorer in chromium towards the metal substrate.
In what precedes and follows, M designates a metal such as iron (Fe), chromium (Cr), nickel (Ni), etc. . . .
The surface skin has a thickness of between 12 and 18 microns, and a level of hardness between 1200 and 1800 on the Vickers hardness scale. Generally, the chromium thus diffuses in the steel to a depth of nearly 15 Microns. In known methods, this chromizing depth never exceeds 20 microns.
The affinity of chromium for carbon is such that it very rapidly forms a skin of carbides of the M.sub.7 C.sub.3 type on the surface of the pieces as the treatment temperature rises. This skin impedes the penetration of the chromium into the steel by diffusion; from this results:
(1) the formation of the second type of carbides M.sub.23 C.sub.6, PA1 (2) the obtaining of relatively thin, surface layers of carbides.
These thin, two-phase layers have the drawback of being relatively brittle, because of the state of stresses existing in the carbide phases after thermal treatment. The carbide M.sub.7 C.sub.3, columnar in structure, especially exists in a state of tensile stress, which gives rise to the forming of cracks which are often the origin of the flaking observed.